1. Field of the Invention
This invention relates to a sacrificial anode and an apparatus employing same for treating hot sea water, and more particularly to a sacrificial anode for use in preventing corrosion of copper alloy parts and hydrogen absorption in titanium or titanium alloy parts, which are built in the apparatus.
2. Description of the Prior Art
Typical of the sea-water desalting processes which have found a wide range of application is a multi-stage flashing process. FIG. 1 shows an outline of a multi-stage flashing process. Shown at 1 is a heat rejection section, at 2 a heat recovery section, and at 3 a sea-water heating section. Sea water A is delievered into heat transfer tubes 4 (see FIG. 2) in the heat rejection section 1 by means of a pump, and then used as a cooling medium for condensation of steam in a series of chambers, so that the sea water used as a coolant gradually or progressively absorbs the heat, as it travels through the apparatus. More particularly, sea water picks up latent heat from steam in the heat recovery section 2 and is further heated, and is then fed to the sea water heating section 3. The sea water is further heated by an external heat source, i.e., by steam present in the sea-water heating section 3, and then delivered past a series of chambers in the heat recovery section 2 and heat rejection section 1, whose pressures are decreased progressively, so that the sea water is subjected to flash-evaporation as it passes through these sections. The sea water thus condensed is discharged in the form of brine B by means of a pump and then introduced in pipe lines in the heat recovery section 2 again, for recirculation. Plain water which has evaporated from sea water in the heat recovery section 2 and heat rejection section 1 and which has clung to the outer surfaces of heat transfer tubes 4 is collected in a tray, and then withdrawn as plain water C by the suction of a pump. In this manner, because the multi-stage-flashing process utilizes heat exchange between heated water, and cooled condensate of evaporated or distilled water, good thermal efficiency is achieved thus rendering the process very desirable industrally. Because of the efficiency of the process increased amounts of sea water are treatable per unit time, thus finding wide application for the process in industry.
The sea-water-transportation tubes positioned in the upper portions of the heat rejection section, heat recovery section 2 and sea-water-heating section 3 are referred to as heat transfer tubes. For instance, as shown in FIG. 2 (a partial perspective view) and FIG. 3 (a partial cross-sectional view of FIG. 2, taken along the line III--III thereof), two or more heat transfer tubes 4 are arranged in parallel to improve heat exchange efficiency. In this respect, the opposite ends of each heat transfer tube 4 are secured to tube plates 5 having holes therein, while the intermediate portions thereof are supported by suitable buffle plates 6 having a plurality of holes therein, so that the oscillation of tubes because of the pulsating pressures of steam and sea water may be prevented. The heat transfer tubes 4 are enclosed within shell 31. Moreover, the inner surface of each heat transfer tube 4 contacts sea water, when sea water passes therethrough, while the outer surface of the heat transfer tube 4 is exposed to steam. For these reasons, superb corrosion resistance characteristics are essential for the heat transfer tubes 4. Accordingly, in general conventional apparatuses employ heat transfer tubes made of titanium, titanium alloys, copper alloys or the like, while the tube plates 5 are made of a titanium-lined steel, copper alloy or the like, and the buffle plates 6 are made of steel. However, apparatus of the type described above have recently been replaced by a combination of heat transfer tubes made of titanium or a titanium alloy, and tube plates 5 made of a copper alloy, because of the increased demands for increased service life and economy.
In general, the titanium alloys which have been available for this purpose include Ti-5Ta, Ti-6Al-4V, Ti-5Al-2Cr-1Fe, Ti-5Al-2.5Sn, Ti-15Mo-5Zr, Ti-0.3Mo-0.8Ni, Ti-15Mo-5Zr-3Al and the like. Suitable copper alloys have included naval brass, aluminum bronze, nickel-aluminum bronze, 9/1 cupronickel, 7/3 cupronickel and the like. At any rate, the use of these alloys, i.e., copper alloys and titanium or titanium alloys which will be simply referred to hereinafter as titanium, as the case may be, presents the problem that the copper alloys and titanium are electrically shortcircuited in hot deaerated sea water or sodium chloride solution, which results in the galvanic corrosion of these alloys. In this respect, however, the titanium heat-transfer tubes 4 are not significantly subject to galvanic corrosion, although various problems are likely to occur because of corrosion to the copper alloy tube plates 5, with the result that the portions of the tube plates 5 which hold the heat transfer tubes 4 come off, or otherwise sea water leakes therethrough, thus failing to provide the intended, long service life for apparatus which treat hot, deaerated sea water. Furthermore, still another problem is the potential danger of hydrogen brittleness of the titanium tubes which results from the absorption of hydrogen by the tubes which stems from the corrosion in the copper alloy parts of the apparatus thus presenting still another cause for premature trouble or loss of intended service life of the titanium tubes.